The Home Office has mistakenly informed approximately 100 people that they are to be deported from the UK.
A letter was issued "in error" to a number of people stating that a decision had been taken to remove them from the UK under immigration law.
It informed recipients that they are "liable to be detained" for failing to provide evidence that they are exercising Treaty rights in the UK.
A Home Office spokesperson has since said they are "urgently" looking into why the letters were wrongly sent out.
The letter, signed by UK Visas and Immigration, states: "A decision has been taken to remove you from the United Kingdom in accordance with section 10 of the Immigration and Asylum Act 1999."
It proceeds state: "Your are therefore a person who is liable to be detained under paragraph 16 of Schedule 2 of the Immigration Act 1971.
One woman who received the letter, Eva Johanna Holmberg, a Finnish historian living in Brighton with her British husband, told The Independent she "couldn't believe what she was seeing" when she opened the document.
But then she tried to contact the Home Office to ask whether it has been an administrative error, Dr Holmberg said she was sent between phone lines, and ultimately advised only to contact her MP.
“It came out of the blue, there’s no reason for it. I tried to correct it by calling the Home Office to find out if it was an administrative error, but they just sent me from phone line to phone line then advised me to call my MP," she said.
“I work here, being seconded by Helinski University, but like many others here I am a visiting academic affiliated with a UK university. This is normal for academics especially if you’re studying British history. It’s vital for my work.
“I opened it and it had the words ‘the decision has been made to remove you’. I couldn’t believe what I was seeing."
When The Independent contacted the Home Office about Dr Holmberg's case, a spokesperson said the letter had been issued "in error", along with around 100 others, and that they are in the process of "urgently looking into" why it occured.
The spokesperson said: “A limited number of letters were issued in error and we have been urgently looking into why this happened. We are contacting everyone who received this letter to clarify that they can disregard it.
“We are absolutely clear that the rights of EU nationals living in the UK remain unchanged.”
Dr Holmberg's local MP, Green Party co-leader Caroline Lucas, who has since contacted the Home Office regarding the situation, accused the Government of "turning lives upside down" by "callously playing hard ball over Brexit".
“It beggars belief that Eva, who is married to a UK citizen, pays taxes in the UK and has five years guaranteed employment as a visiting fellow, has been issued with a letter threatening her removal and possible detention," she said.
“I am fighting this decision on her behalf and on behalf of all those living in the UK whose lives are being turned upside down by the Government callously playing hard ball over Brexit.”
In response to the Home Office error, Liberal Democrat Shadow Home Secretary Ed Davey meanwhile said: “These letters shame Britain. EU nationals who have made their lives here are already facing huge uncertainty over Brexit.
“It is appalling that some are now being officially threatened with deportation. This catastrophic error is a sign of the chaos and incompetence at the heart of this Conservative government.
“Amber Rudd should personally write to apologise to each of those affected and ensure they are fully reimbursed for any legal costs incurred.”
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